1. Field of the Invention
This invention relates to aqueous dispersions of sequentially-produced polymer particles especially useful for binders, coatings, and adhesives, to processes for preparation thereof, and to the uses.
2. Description of the Prior Art
The use of sequential emulsion polymerization to produce dispersions of film-forming core-sheath or core-shell polymer particles is very well known. Many references teach the use of such dispersions in binders, coatings, and adhesives, as film-forming polymers.
The present invention also utilizes sequential emulsion polymerization to produce film-forming binders, coatings and adhesives and non-film forming additives to coatings but, contrary to conventional technology, the particles of this invention are not core-shell or core-sheath or even spherical. We have discovered how to produce particles having a central core surrounded by lobes which we refer to as "multilobals"; and have also discovered how to use this phenomenon to obtain advantages in coating film build, flow, leveling, high shear ("ICI") viscosity, and adhesive wet tack.
We have not been able to find multilobals in the published prior art, nor are we aware of any prior use or sale of multilobals.
We are aware that bumpy, lumpy, or "raspberry" like particles are sometimes present in conventional polymer dispersions in water. We are also aware that dispersions of conventional spherical particles sometimes aggregate to form clusters of elliptical or dumbbell-shaped particles. Our multilobal dispersions are quite different in structure and behavior in binder, coating, or adhesive compositions. Structurally our multilobal particles have a central core surrounded by lobes, the lobes and core having different compositions. In preferred systems our core is swellable upon neutralization and has a difference in composition with the lobe composition defined by an interaction parameter, X.sub.C-L, greater than 0.05.
One prolific group at Kobe University, let by Masayoshi Okubo, has published many articles over the last twelve years in the field of suspension and emulsion polymerization, and has been especially interested in what they refer to as anomalous and peculiar morphology. See for example Journal of Polymer Science: Polymer Letters Edition, Vol. 20, 45-51 (1982), and Vol. 19, 143 (1981). While some of the Kobe articles show what appear to be lobes, upon closer study we believe the Kobe group is growing hard second stage particles, e.g., styrene within a continuous soft first stage, e.g., butyl acrylate. Since butyl acrylate is transparent to an electron beam, transmission electron microscopy photos of unhardened particles show a bumpy internal morphology. However, the Kobe particles are actually spherical when viewed by optical microscope or even by electron microscope after hardening the particles. Certainly the Kobe materials would not be useful as high film build coating binders or high wet tack adhesives. The Kobe materials are made by a method which is different in many regards from ours, and have a different morphology from our multilobals. Rather than having a central core surrounded by lobes, we believe the Kobe particles have a bumpy core surrounded by a spherical but transparent (to electron beam) shell.
Conventional dispersions generally need a thickener to produce binders, coatings, or adhesives with sufficient film build and ICI viscosity. Typical thickeners used in the art are hydroxyethyl cellulose and synthetic rheology modifiers.